As a general rule, the approach of the tool towards the workpiece in machine tools is effected at a relatively high speed in order to reduce as much as possible the dead time of the machine. However, when the tool comes into contact with the workpiece, it is desirable to reduce very rapidly such high speed of advance in order to avoid deterioration as much in the tool itself as in the workpiece.
There have long been known arrangements, employed particularly in grinders or truers, which include an acoustic sensor, preferably of the piezo-electric type, intended to detect an acoustic emission which is generated when the grinder comes into contact with the workpiece, the signal thus obtained being employed following amplification thereof to reduce, if necessary to zero, the relatively high speed employed during the approach phase.
Such an arrangement is described for instance in Swiss Pat. No. 585,609 where the sensor is fixed to the framework of the machine or, more exactly, on the carrier of the workpiece.
A similar arrangement is found in French Pat. No. 2,382,310. Here a vibration sensor is mounted on the workpiece support in a machine of the centerless type.
At the same time, the known arrangements exhibit a difficulty which consists in that the inherent noise generated by the tool when it comes into contact with the workpiece, is scarcely different in intensity and in frequency from the ambient noises produced by the machine, for example the noises of the bearings, gears, pneumatic or hydraulic driving elements, etc. to such an extent that satisfactory operation of the arrangement may not always be assured.
It has likewise been suggested to mount the sensor on one of the measuring feelers of a self-calibrating system, said feelers being in direct contact with the workpiece. However, this contact is very localized if one wishes to measure precisely the diameter of the workpiece, from whence it follows that the feelers transmit poorly the noise which is sought to be detected. Furthermore, on certain machines provided with a self-calibrating system, the setting of said feelers is delayed in order to prevent their wear. It is thus not certain that such feelers are in contact with the workpiece at the moment of contact of the tool part.